The invention relates to carbon nanomaterial production in sooting flames, in particular burners, combustion apparatus, and methods for carbon nanomaterial production.
The term “carbon nanomaterials” is used generally herein to refer to any substantially carbon material containing six-membered rings that exhibits curving of the graphite planes, generally by including five-membered rings amongst the hexagons formed by the positions of the carbon atoms, and has at least one dimension on the order of nanometers. Examples of carbon nanomaterials include, but are not limited to, fullerenes, single-walled carbon nanotubes (SWNTs), multiple-walled carbon nanotubes (MWNTs), nanotubules, and nested carbon structures with dimensions on the order of nanometers. The term “fullerene” is used generally herein to refer to any closed cage carbon compound containing both six- and five-member carbon rings independent of size and is intended to include the abundant lower molecular weight C60 and C70 fullerenes, larger known fullerenes including C76, C78, C84 and higher molecular weight fullerenes C2N where N is 50 or more. The term is intended to include “solvent extractable fullerenes” as that term is understood in the art (generally including the lower molecular weight fullerenes that are soluble in toluene or xylene) and to include higher molecular weight fullerenes that cannot be extracted, including giant fullerenes which can be at least as large as C400. Carbon nanomaterials may be produced in soot and, in certain cases, carbon nanomaterials may be isolated from the soot or enriched in the soot. Soot produced during the synthesis of carbon nanomaterials, such as fullerenes, typically contains a mixture of carbon nanomaterials which is a source for further purification or enrichment of carbon nanomaterials or which may itself exhibit desired properties of carbon nanomaterials and be useful as an addition to convey those properties. The term “carbon nanomaterials,” when used without limitation, is intended to include soot containing detectable amounts of carbon nanomaterials. For example, the term fullerenic soot is used in the art to refer to soot containing fullerenes. Fullerenic soot is encompassed by the term carbon nanomaterials.
Different carbon nanomaterials have different potential applications. Fullerenes and fullerenic soot have potential applications as additives to electron- and photo-resists for semiconductor processing; for use in proton-conducting membranes for fuel cells, optical limiting materials and devices, and lithium battery anodes; as active elements in organic transistors; as pigments in cosmetics; as antioxidants; and as therapeutics, e.g., as anti-viral agents. While the art recognizes significant potential for commercial application of carbon nanomaterials, the high cost and difficulty in obtaining these materials in the large amounts necessary for developing these applications has been a major impediment in practical application of these materials.
Sooting flames are the most cost-effective way to produce carbon nanomaterials at large production rates (greater than roughly 100 g/day). Fullerene synthesis in premixed flames stabilized on a water-cooled flat metal plate, where the plate forms the outlet for the gases and the surface of the burner, is known to the art. (Howard et al., U.S. Pat. No. 5,273,729). This kind of burner was developed for combustion research studies, not for materials production. However, burners with water-cooled surfaces work only over a relatively narrow range of operating parameters. Also, burners with water-cooled surfaces sink a large portion of the heat generated by burning the hydrocarbon into the cooling water, rather than using the heat in the fullerene-forming reactions. Further, use of a cooled burner surface results in increased deposit formation on the burner surface, causing irregularities in gas flow, leading to inhomogeneities in the flame, and adversely affecting the material production yield and homogeneity. Eventually, the burner surface becomes coated, and the synthetic process must be stopped to clean the burner.
An uncooled burner surface which can be operated at higher temperatures has several advantages for fullerene production. The rate of buildup of fullerene deposits on the burner plate is dramatically reduced, because the deposits are more readily volatilized or burned off. As a result, the uncooled burners require cleaning less often, if ever. It is also more efficient to operate an uncooled burner, because the heat load on the burner can heat the gas flow, raising the flame temperature. The chemical energy released by combustion is used more efficiently rather than being lost to the cooling water. With an uncooled burner surface, combustion is anchored right at the surface, making it more difficult to blow out the flame as velocities are increased, significantly increasing flame stability. Thus, the increased flame stability that results from the use of the uncooled burner allows for higher throughput. Another advantage of an uncooled burner surface is the ability to introduce low vapor pressure additives into the flame as gases, without condensing on the cool burner plate surface. One example of such additives are high-boiling polycyclic aromatic hydrocarbon (PAH)-rich feedstocks that serve as cost-effective, high-yield feeds for fullerene production. PAHs are aromatic hydrocarbon molecules containing two or more six-membered rings, two or more five-membered rings or a mixture of one or more five- and one or more six-membered rings. Other examples are catalysts that sublime at elevated temperatures, easing their incorporation into the feed stream.
Burners with high temperature (uncooled) surfaces have been used for applications other than carbon nanomaterial synthesis such as industrial furnaces. For example, Abe et al., U.S. Pat. No. 4,673,349 describe a high temperature surface combustion burner having a porous ceramic body. In both embodiments of the invention reported, the porous ceramic body contains throughholes. U.S. Pat. No. 4,889,481 to Morris et al. reported a dual structure porous ceramic burner for use as an infrared heat source. U.S. Pat. No. 5,470,222 to Holowczack et al. reported a high emissivity porous ceramic flame holder for use in a heating unit.
It is, however, known in the art that special fuels and combustion conditions are required for production of substantial amounts of fullerenes. During normal or industrial combustion the formation of fullerenes is so low that these materials can only be detected with the most sensitive analytical techniques (K.-H. Homann, Angew. Chem. Int. Ed. 1998, 37, 2434-2451). Burner design is one of the variables that must be optimized to provide efficiency and an acceptable rate of fullerene production (A. A. Bogdanov et al, Technical Physics, Vol. 45, No. 5, 2000, pp. 521-527). Many of the conditions required for high fullerene yields by combustion are extremely unusual, and the combination of conditions is unique. In contrast to burners that are used for heat generation or propulsion, burners used for fullerene synthesis produce copious quantities of solid carbon product. Further, highly aromatic fuels instead of hydrocarbon gases such as alkanes, are the optimum feeds for fullerene production and the feedstock is preferably burned in oxygen rather than air. Another unique aspect of fullerene production by combustion is that the burner is preferably operated at sub-atmospheric pressure. Implicit in the use of low pressures is low Reynolds number (typically less than 100) burner operation, in contrast to high Reynolds number operation (typically in the thousands) for burners used in other applications. Low Reynolds number operation implies extremely laminar flow and mixing of gases solely by molecular diffusion. A burner intended for fullerene production should be designed for optimal performance under these unique operating conditions.
There is a need for improved methods and apparatus, including burners, for making carbon nanomaterials, including fullerenes, in sooting flames which can lower the cost of production and provides these materials in sufficient quantities for practical application.